The letters, which lay undiscovered and away from public view in the Polish national library until they were shown to the BBC's Panorama programme, tell of a complex and intense friendship in which it has been suggested that Ms Tymieniecka told the pontiff she loved him.

In the correspondence, Pope John Paul II declares she is a "gift of God", indicating he was struggling to come to terms with the nature of their relationship. He writes: "If I did not have this conviction, some moral certainty of Grace, and of acting in obedience to it, I would not dare act like this."

The letters also reveal he gave her the scapular (a small devotional necklace) his father had given him at his first Holy Communion.

There is no suggestion his vow of celibacy was broken at any point.

Their correspondence began when Ms Tymieniecka contacted him in 1973 about a book of philosophy he had written.

Eamon Duffy, professor of the History of Christianity at Cambridge University, said: "Here is one of the handful of transcendentally great figures in public life in the 20th Century, the head of the Catholic Church, in an intense relationship with an attractive woman."

He wrote in September 1976: "My dear Teresa, I have received all three letters.

"You write about being torn apart, but I could find no answer to these words."

The correspondence can only be understood so far, as the only letters available are the ones he sent to her.

The researchers analysing his letters believe that in the early stage of their relationship, around 1975, Anna-Teresa Tymieniecka told him she was in love with him.

In another letter from September 1976, he explains the gift of the scapular. "Already last year I was looking for an answer to these words, 'I belong to you', and finally, before leaving Poland, I found a way - a scapular.

"The dimension in which I accept and feel you everywhere in all kinds of situations, when you are close, and when you are far away."

Even after he became Pope, the two continued their correspondence and she visited the Vatican on various occasions.

Carl Bernstein, the American journalist famous for uncovering the Watergate scandal was the first person to write about the significance of the relationship in the 1990s.

He said: "We are talking about Saint John Paul. This is an extraordinary relationship.